ASTRO-H is the 6th Japanese-led international X-ray observatory, and the successor to the Suzaku satellite (ASTRO-E2, launch July 10, 2005) which is operational in 2016. The ASTRO-H mission was formerly known as NeXT (New exploration X-Ray Telescope). The ASTRO-H mission will open a new observing window through the first imaging spectroscopy in the hard X-ray band above 10 keV, and also achieve unprecedented observing capability in the soft X-ray band below 10 keV. 1)2)3)4)5)6)7)8)

The Astro-H mission objectives are to:

• trace the growth history of the largest structures in the Universe

• provide insights into the behavior of material in extreme gravitational fields

• determine the spin of black holes and the equation of state of neutron stars

ASTRO-H enables high sensitivity observations of celestial sources across a wide energy range, from X-rays to gamma-rays, bands presenting considerable technical challenges. The satellite features cutting-edge instruments; SXS (Soft X-Ray Spectrometer), operated at only 50 mK, is capable of measuring, with unprecedented accuracy, the energy of incoming X-rays. It measures temperature changes in a sensor resulting from absorption of X-ray photons. The HXI (Hard X-ray Imager) will produce the first ever images of the high-energy X-ray universe. SXI (Soft X-ray Imager), featuring domestically produced X-ray CCDs, will enable the project to make wide field X-ray images of the sky with ultra-low noise. The narrow view semi-conductor Compton camera, SGD (Soft Gamma-ray Detector), revitalizes the field of gamma-ray observations by featuring the greatest sensitivity in this band. The Japanese heritage of successful previous satellites will provide a basis for meeting these challenges. 9)

ASTRO-H international cooperation:

• ASTRO-H is a joint JAXA (Japan) and NASA (USA) astronomy mission with the objective to study the extreme environments of the universe. The agreement calls for JAXA to provide the main spacecraft and several instruments, while NASA/GSFC will be adding a new instrument to the spacecraft, the high resolution SXS. The main instrument of the mission will be the HTX (Hard X-ray Telescope). However, the addition of SXS is just one of several complementary instruments that provide a "yin and yang" aspect to NeXT's explorations, the aim is to reveal new facets of the universe.

• CSA (Canadian Space Agency) is also contributing to the ASTRO-H mission, providing CAMS (Canadian ASTRO-H Metrology System), which is an innovative measuring system that will help better calibrate the observatory's main telescope and significantly enhance the images it captures. In April 2014, CSA awarded a contract to the Neptec Design Group of Ottawa to continue work on Canada's contribution to ASTRO-H. 10)11)

• In October 2009, SRON (Netherlands Institute for Space Research) and JAXA signed a cooperative framework agreement with the aim of promoting joint activities in the field of space science. SRON, together with the University of Geneva, will provide filters, which will control X-ray flux during observing bright X-rays sources, and a "filter wheel" that selects an appropriate filter. This instrument will be located between the X-ray telescope and the SXS (Soft X-ray Spectrometer) instrument, which enables ASTRO-H to achieve the proper spectroscopic performance. The filters will enable scientists to study the space-time structure in the close vicinity of black holes. The filter wheel will carry calibration X-ray sources to monitor the X-ray energy determination accuracy of the detectors. 12)13)

• ESA (European Space Agency) is providing: Three Science Advisors;Contribution to mission instruments (SXS/HXI/SGD/HXT); User support in Europe. A cooperation agreement with JAXA was approved by the 132nd ESA Science Program Committee Meeting in February 2011.

• CEA (French National Atomic Energy Commission)/DSM (Physical Science Division)/IRFU (Institute of Research into the Fundamental Laws of the Universe) of France: Contribution to BGO Shield/ASIC test.

The collaboration encompasses 58 institutions (Japan 33), some 266 scientists and leading engineers are involved in the ASTRO-H mission, they are from Japan, USA, Europe, and Canada. 14)

There are four focusing telescopes mounted on the top of a fixed optical bench (FOB). Two of the four telescopes are SXTs (Soft X-ray Telescopes) with a focal length of 5.6 m. They will focus medium-energy X-rays (E ~ 0.3-12 keV) onto focal plane detectors mounted on the base plate of the spacecraft.

One SXT will point to a micro-calorimeter spectrometer array with excellent energy resolution of 57 eV, and the other SXT will point to a large-area CCD array. The other two telescopes are HXTs (Hard X-ray Telescopes) capable of focusing high-energy X-rays (E = 5-80 keV). The focal length of the HXTs is 12 m. The HXIs (Hard X-ray Imaging) detectors are mounted on the HXI plate, at the end of a 6 m EOB (Extendable Optical Bench) that is stowed to fit in the launch fairing and deployed once in orbit. In order to extend the energy coverage to the soft γ-ray region up to 600 keV, the SGD (Soft Gamma-ray Detector) will be implemented as a non-focusing detector. Two SGD detectors, each consisting of three units will be mounted separately on two sides of the satellite. With these instruments, ASTRO-H will cover the entire bandpass between 0.3 keV and 600 keV.

The lightweight design of the EOB renders it vulnerable to distortions from thermal fluctuations in LEO (Low Earth Orbit) and spacecraft attitude maneuvers. Over the long exposures associated with X-ray observing, such fluctuations might impair HXI image quality unless a compensation technique is employed. To provide the required corrections, the Canadian contribution to the the ASTRO-H project is a laser metrology system CAMS (Canadian ASTRO-H Metrology System) that will measure displacement in the alignment of the HXT optical path. The CAMS consist of a laser and detector module (CAMS-LD) located on the top plate of the FOB, and a passive target module (CAMS-T) consisting of a retroreflector (corner cube mirror) mounted on the EOB detector plate (HXI plate).

The spacecraft attitude is stabilized by four sets of reaction wheels with one redundancy, while the attitude is measured by two star trackers and its change rate by two gyroscopes. There are two more gyroscopes mounted in skew directions, which provide redundancy. The accumulated angular momentum is unloaded by magnetic torquers that interact with the Earth's magnetic field. The required accuracy of the spacecraft attitude solution is approximately 0.33 arcmin with a stability of better than 0.12 arcmin per 4s (a nominal exposure time for the CCDs). The pointing direction of the telescope is limited by the power constraint of the solar panels. The area of the sky accessible at any time is a belt within which the Sun angle is between 60º and 120º. Any part of the sky is accessible at least twice a year. It is expected to take ~72 minutes for a 180º maneuver.

Almost all of onboard subsystems of ASTRO-H, such as the command/data handling system, the attitude control system, and four types of X-ray/gamma-ray telescope instruments, are connected to the SpaceWire network using a highly redundant topology. The number of physical SpaceWire links between components exceeds 140 connecting ~40 separated components (i.e., separated boxes), and there are more links in intra-component (intra-board) networks. Most of the electronics boxes of both the spacecraft bus and the scientific instruments are mounted on the side panels of the space craft. The electronics boxes for the HXI are mounted on the HXI plate.

The ASTRO-H spacecraft has a mass of ~2700 kg, a total height of 14 m, power of <3.5 kW, telemetry rate of 8 Mbit/s (X-band, QPSK modulation), an onboard recording capacity of 12 Gbit at EOL, and a design life of > 3 years.

• The ASTRO-H project officially started at JAXA in October 2008. The PDR (Preliminary Design Review) was held in May 2010. After the detailed design phase (Phase C) was completed, the design of the satellite was reviewed and passed the first CDR1 (Critical Design Review) which was held in February 2012.

• Since the thermal and mechanical design had to be verified well in advance, the project started from manufacturing the spacecraft structure, which includes the side panels, baseplates, and the FOB as pre-FM (Flight Module) components. These components were assembled to form the TTM (Thermal Test Model) and STM (Structural Test Model) of the satellite.

Figure 4: ASTRO-H timeline (image credit: JAXA, T. Takahashi)

• A series of test campaigns were carried out to ensure the validity of thermal design and mechanical design of the structure. In addition to usual thermal, acoustic, and vibration tests, the project performed a dedicated thermal deformation test to verify the correctness of the design with respect to the alignment requirements for all co-aligned telescopes and instruments onboard ASTRO-H. The thermal deformation during the ground test should be measured with an accuracy better than 5 µm and 2 arcseconds for the size of the structure of about 10 meters. To perform the thermal deformation test with such a high accuracy, a novel technique was developed and applied to ASTRO-H.

• By using most of FM components, the first integration test campaign started in August 2013 (Figure 5). The electrical and mechanical interfaces between the satellite bus and the subsystem components, were verified, together with the electrical power system of the satellite. In June 2014, the project successfully completed the test campaign. All components mounted on the space craft structure are now being disassembled for the final preparation. After the project team refurbish them, the final functional testing of each subsystem will be performed. For scientific instruments, the final calibration will also be carried out. The final integration and testing will start in November 2014.

Figure 5: Photo taken at the first integration test in April 2014 (image credit: JAXA)

Launch: The ASTRO-H satellite was launched on February 17, 2016 (08:45:00 UTC) from the Yoshinobu Launch Complex at TNSC (Tanegashima Space Center) on the H-IIA F30 vehicle. The launch provider was MHI (Mitsubishi Heavy Industries, Ltd.). The launch vehicle flew as planned, and at approximately 14 minutes and 15 seconds after liftoff, the separation of ASTRO-H was confirmed. 15)16)

ASTRO-H is the eye to study the hot and energetic universe. Therefore, after launch JAXA nicknamed ASTRO-H "Hitomi". The word Hitomi generally means "eye", and specifically the pupil, or entrance window of the eye - the aperture! 17)

Figure 7: The three smaller satellites were carried on the Second Stage Adapter ring (image credit: JAXA)

Science operations:

The science operations will be similar to those of Suzaku (ASTRO-E2), with pointed observation of each target until the integrated observing time is accumulated, and then slewing to the next target. A typical observation will require a few x 100 ksec integrated exposure time. All instruments are co-aligned and will operate simultaneously.

ASTRO-H is in many ways similar to Suzaku in terms of orbit, pointing, and tracking capabilities. After the satellite is launched, the current plan is to use the first three months for check-out and start the PV phase with observations proprietary to the ASTRO-H team. GO (Guest Observing) time will start from 10 months after the launch. About 75% of the satellite time will be devoted to GO observations after the PV phase is completed. The project is planning to implement key-project type observations in conjunction with the GO observation time.

The telemetry from the satellite is downloaded and stored at the ground stations. The telemetry is distributed in realtime to the operation control unit and quicklook systems via the SDTP (Space Data Transfer Protocol) at the ground station. Then, the telemetry stored on each station is transferred to the SIRIUS database in JAXA. In the pre-pipeline process, the raw data are extracted from the SIRIUS database via SDTP protocol and stored into several FITS (Felxible Image Transfer System) files, RPT (Raw Packet Telemetry), ATT (Attitude file), ORB (Orbit file), and CMD (Command) log.

The RPT contains all the information from the satellite. Since the RPT is just a dump of the space packets, the file is converted into the FFF (First FITS Files), which contain the meaning of the attributes of the onboard instruments. The data processing script run conversion of the FFF into calibrated event files, the so-called SFF (Second FITS File), corresponding to the Level 1 or unfiltered file, applies the data screening generating the Level 2 or cleaned files, and if appropriate extracts the Level 3 or data products such as spectra light curves and others. It also creates the so-called make filter file. All data files are in the FITS format and the software used in the pipeline is included in the standard ASTRO-H software package distributed to the science community. The data files archived include the Level 1, 2 and 3, corresponding to the unfiltered, cleaned and product files, the housekeeping data, orbit and attitude and the make filter file.

Mission status:

• November 13, 2017: Before its brief mission ended unexpectedly in March 2016, Japan's ASTRO-H/Hitomi X-ray observatory captured exceptional information about the motions of hot gas in the Perseus galaxy cluster. Now, thanks to unprecedented detail provided by an instrument developed jointly by NASA and JAXA (Japan Aerospace Exploration Agency), scientists have been able to analyze more deeply the chemical make-up of this gas, providing new insights into the stellar explosions that formed most of these elements and cast them into space. 18)

- The Perseus cluster, located 240 million light-years away in its namesake constellation, is the brightest galaxy cluster in X-rays and among the most massive near Earth. It contains thousands of galaxies orbiting within a thin hot gas, all bound together by gravity. The gas averages 50 million degrees Celsius and is the source of the cluster's X-ray emission.

- Using Hitomi's high-resolution SXS (Soft X-ray Spectrometer) instrument, researchers observed the cluster between Feb. 25 and March 6, 2016, acquiring a total exposure of nearly 3.4 days. The SXS observed an unprecedented spectrum, revealing a landscape of X-ray peaks emitted from various chemical elements with a resolution some 30 times better than previously seen.

- In a paper published online in the journal Nature on Nov. 13, the science team shows that the proportions of elements found in the cluster are nearly identical to what astronomers see in the Sun. 19)

- "There was no reason to expect that initially," said coauthor Michael Loewenstein, a UMD (University of Maryland) research scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "The Perseus cluster is a different environment with a different history from our Sun's. After all, clusters represent an average chemical distribution from many types of stars in many types of galaxies that formed long before the Sun."

Figure 8: Hitomi's SXS (Soft X-ray Spectrometer) instrument captured data from two overlapping areas of the Perseus galaxy cluster (blue outlines, upper right) in February and March 2016. The resulting spectrum has 30 times the detail of any previously captured, revealing many X-ray peaks associated with chromium, manganese, nickel and iron. Dark blue lines in the insets indicate the actual X-ray data points and their uncertainties (image credit: NASA/GSFC)

- One group of elements is closely tied to a particular class of stellar explosion, called Type Ia supernovas. These blasts are thought to be responsible for producing most of the universe's chromium, manganese, iron and nickel — metals collectively known as "iron-peak" elements.

- Type Ia supernovas entail the total destruction of a white dwarf, a compact remnant produced by stars like the Sun. Although stable on its own, a white dwarf can undergo a runaway thermonuclear explosion if it's paired with another object as part of a binary system. This occurs either by merging with a companion white dwarf or, when paired with a nearby normal star, by stealing some of partner's gas. The transferred matter can accumulate on the white dwarf, gradually increasing its mass until it becomes unstable and explodes.

- An important open question has been whether the exploding white dwarf is close to this stability limit — about 1.4 solar masses — regardless of its origins. Different masses produce different amounts of iron-peak metals, so a detailed tally of these elements over a large region of space, like the Perseus galaxy cluster, could indicate which kinds of white dwarfs blew up more often.

- "It turns out you need a combination of Type Ia supernovas with different masses at the moment of the explosion to produce the chemical abundances we see in the gas at the middle of the Perseus cluster," said Hiroya Yamaguchi, the paper's lead author and a UMD research scientist at Goddard. "We confirm that at least about half of Type Ia supernovas must have reached nearly 1.4 solar masses."

- Taken together, the findings suggest that the same combination of Type Ia supernovas producing iron-peak elements in our solar system also produced these metals in the cluster's gas. This means both the solar system and the Perseus cluster experienced broadly similar chemical evolution, suggesting that the processes forming stars — and the systems that became Type Ia supernovas — were comparable in both locations.

- "Although this is just one example, there's no reason to doubt that this similarity could extend beyond our Sun and the Perseus cluster to other galaxies with different properties," said coauthor Kyoko Matsushita, a professor of physics at the Tokyo University of Science.

- Although short-lived, the Hitomi mission and its revolutionary SXS instrument —developed and built by Goddard scientists working closely with colleagues from several institutions in the United States, Japan and the Netherlands — have demonstrated the promise of high-resolution X-ray spectrometry.

- "Hitomi has permitted us to delve deeper into the history of one of the largest structures in the universe, the Perseus galaxy cluster, and explore how particles and materials behave in the extreme conditions there," said Goddard's Richard Kelley, the U.S. principal investigator for the Hitomi collaboration. "Our most recent calculations have provided a glimpse into how and why certain chemical elements are distributed throughout galaxies beyond our own."

- JAXA and NASA scientists are now working to regain the science capabilities lost in the Hitomi mishap by collaborating on the XARM (X-ray Astronomy Recovery Mission), expected to launch in 2021. One of its instruments will have capabilities similar to the SXS flown on Hitomi.

- Hitomi launched on Feb. 17, 2016, and suffered a mission-ending spacecraft anomaly 38 days later. Hitomi, which translates to "pupil of the eye," was known before launch as ASTRO-H. The mission was developed by ISAS (Institute of Space and Astronautical Science), a division of JAXA. It was built jointly by an international collaboration led by JAXA, with contributions from Goddard and other institutions in the United States, Japan, Canada and Europe.

• July 06, 2016: With its very first observation, the ASTRO-H (Hitomi) X-ray observatory has discovered that the gas in the Perseus cluster of galaxies is much less turbulent than expected. This is a surprise because the Perseus cluster is home to NGC 1275, a highly energetic active galaxy. 20)21)22)

- The result allows the mass of the Perseus cluster to be calculated more accurately than before. Once this technique can be extended to other clusters, it will allow cosmologists to use them as better probes of our models of the Universe's evolution from the Big Bang to the present time.

- Hitomi (originally known as ASTRO-H) is the sixth in a series of Japanese X-ray observatories. Led by JAXA (Japan Aerospace Exploration Agency), it is a collaboration of over 60 institutes and 200 scientists and engineers from Japan, the US, Canada, and Europe. The spacecraft was launched on 17 February 2016 from the Tanegashima Space Center, Japan.

- "Hitomi targeted the Perseus cluster just a week after it arrived in space," says Matteo Guainazzi, ESA Hitomi Resident Astronomer at ISAS (Institute of Space and Astronautical Science), Japan. "Perseus is the brightest X-ray galaxy cluster in the sky. It was therefore the best choice to fully demonstrate the power of the SXS (Soft X-ray Spectrometer), an X-ray micro-calorimeter that promised to deliver an unprecedented accuracy in the reconstruction of the energy of the incoming X-ray photons." Waiting astronomers were not disappointed.

- The Hitomi collaboration found that SXS could measure the turbulence in the cluster to a precision of 10 km/s. But it was the absolute velocity of the gas that took them by surprise. It was just 164 ± 10 km/s along the line-of-sight. The previous best measurement for Perseus was taken with ESA's XMM-Newton X-ray observatory. Using a different type of spectrometer, it could only constrain the speed to be lower than 500 km/s.

- Hitomi's measurement is therefore much more precise than any similar measurements performed in X-rays so far. "This is due to the outstanding performance and stability of the SXS in space. This demonstrates that the technology of X-ray micro-calorimeters can yield truly transformational results," says Guainazzi.

- The result indicates that the cluster gas has very little turbulent motions within. Turbulent motions in a fluid are part of our everyday life, as airplane passengers, swimmers, or parents filling a bathtub all experience. The study of such chaotic behavior is also a powerful tool for astronomers to understand the behavior of celestial objects.

- Turbulent energy in Perseus is just 4% of the energy stored in the gas as heat. This is extraordinary considering that the active galaxy NGC 1275 sits at the heart of the cluster. It is pumping jetted energy into its surroundings, creating bubbles of extremely hot gas. It was thought that these bubbles induce turbulence, which keeps the central gas hot.

• June 15, 2016: Due to the anomaly experienced with X-ray Astronomy Satellite ASTRO-H (Hitomi), three of the Japan Aerospace Exploration Agency's executive employees have decided to take a 10% pay cut to their monthly salary for four months, to be effective July 2016. The affected employees are as follows: 23)

• April 28, 2016: JAXA established emergency headquarters and has been doing its utmost to understand the anomaly of ASTRO-H/ Hitomi mission. So far, the project made every effort to confirm the status of ASTRO-H and to regain its functions. Unfortunately, based on the rigorous technical investigation, the following conclusions were made: 24)

1) Most of the project's analyses, including simulations on the mechanisms of object separation, it is highly likely that both solar array paddles had broken off at their bases where they are vulnerable to rotation.

2) Originally, the project had some hopes to restore communication with ASTRO-H, since we assumed we received signals from ASTRO-H three times after object separation. However, the project came to the conclusion that the received signals were not from ASTRO-H due to the differences in frequencies as a consequence of technological study.

- JAXA has also received information from several overseas organizations dealing with the separation of the two solar array paddles from ASTRO-H. Considering this information, the project has determined that the ASTRO-H functions cannot be restored. Accordingly, JAXA will cease its efforts to restore ASTRO-H operations and will focus on the investigation of possible anomaly causes. The project will carefully review all phases from design, manufacturing, verification, and operations to identify the causes that may have led to this anomaly including background factors.

- JAXA expresses its deepest regret for the fact to discontinue the operations of ASTRO-H and extends the most sincere apologies to everyone who has supported ASTRO-H, believing in the excellent results ASTRO-H would bring, to all overseas and domestic partners including NASA, and to all foreign and Japanese astrophysicists who were planning to use the observational results from ASTRO-H for their studies.

- JAXA also would like to take this opportunity to send profound appreciation to all overseas and domestic organizations for all of their help in confirming the status of ASTRO-H through ground-based observations and other means.

- Today, JAXA will report the presumptive mechanism of the series of flow from "normal status" to "attitude anomaly", and "objects separation".

- JAXA plans to investigate the mechanism of "communication anomaly".

- JAXA is also investigating background factors that lead the incidents above.

(1) On March 26th, attitude maneuver to orient toward an active galactic nucleus was completed as planned.

(2) After the maneuver, unexpected behavior of the ACS (Attitude Control System) caused incorrect determination of its attitude as rotating, although the satellite was not rotating actually. In the result, the Reaction Wheel (RW) to stop the rotation was activated and lead to the rotation of satellite. [Presumed Mechanism 1]

(3) In addition, unloading (Unloading:Operation to decrease the momentum kept in RW within the range of designed range) of angular velocity by Magnetic Torquer operated by ACS did not work properly because of the attitude anomaly. The angular momentum kept accumulating in RW. [Presumed Mechanism 2]

(4) Judging the satellite is in the critical situation, ACS switched to SH (Safe Hold) mode, and the thrusters were used. At this time ACS provided atypical command to the thrusters by the inappropriate thruster control parameters. As a result, it thrusted in an unexpected manner, and it is estimated that the satellite rotation was accelerated. [Presumed Mechanism 3]

(5) Since the rotation speed of the satellite exceeded the designed speed, the satellites parts that are vulnerable to the rotation such as SAP (Solar Array Paddles), EOB (Extensible Optical Bench) and others separated off from the satellite. [Presumed Mechanism 4]

• After the attitude maneuver operation was completed, ASTRO-H started using STT output data. At that time IRU bias rate estimation* becomes larger than actual one. After that by the correction using STT data, the value converges within normal one.

• There is a possibility that after the end of the attitude maneuver operation on March 26, STT output data had not been uploaded to ASTRO-H for some reason, IRU bias rate estimation remains larger and continued showing anomalous value.

• In this case ACS of ASTRO-H did not use STT output data, and determined the attitude only using IRU. So it is estimated that the attitude was controlled based on the false determination value of attitude.

- ACS of ASTRO-H is designed so that the attitude will controlled only by IRU without loaded STT output data, if the difference between IRU estimated attitude and STT estimated value is larger than 1º.

• From the fact that IRU bias rate estimation mainly around Z axis continuously showed 21.7º/h, it is estimated that attitude control system controlled the attitude to counteract the estimated value with the rotate motion about 21.7º/h around Z axis, then ASTRO-H started rotating.

• By the further analysis of the telemetry received at MSP and MGN, JAXA confirmed that ASTRO-H was rotating around Z axis about 20 º/h.

• As shown in the presumed mechanism 1, it is estimated that ASTRO-H made incorrect determination of its attitude as rotating, although the satellite was not rotating actually. In the result, the RW (Reaction Wheel) to stop the rotation was activated and lead to the rotation of satellite.

• On the other hand, the ACS does not use the sun sensor to determine its attitude, and anomaly was not able to be detected. As the result, the rotation continued.

- The ACS is designed to detect its attitude anomaly by estimated value calculated by the system software. A sun sensor is not used for this purpose.

• At this time, it is presumed that the unloading process of angular momentum in RW by Magnetic Torquer operating in parallel to the rotation control did not work properly because of the attitude anomaly, then angular momentum was accumulated in RW.

• It is confirmed that, by the further analysis of the telemetry data of MGN at 09:50-10:04, the angular momentum in RW was rising near the design limitation (Telemetry 112 Nms, Limitation: 120 Nms)

Table 4: Presumed Mechanism 2: From the attitude anomaly to the continuously rotation of attitude

2: Presumed Mechanism from "Normal Status" to "Objects Separation"

• When exceeding the angular momentum limitation (120 Nms) accumulated in the RW, the ACS concluded that there was anomaly in the control by the RW, then shifted to a mode that controls its attitude using thrusters (Thruster Safe Hold Mode: RCS(Reaction Control System) SH(Safe Hold)).

• In the RCS SH, the satellite conducts the attitude recovery operation using thrusters by detecting the Sun, but it is estimated that there was injection control anomaly with inappropriate RCS control parameter.

• As the result, the velocity of the rotation increased.

• The sequence of events to set the RCS control parameter:

- Feb. 17, by pre-launch RCS control parameter setting, the Sun acquisition control using the thruster right after the launch was conducted normally.

- Feb. 28, JAXA sent the commands to update the RCS control parameter based on the center of mass changes by deployment of the EOB.

- Through the post-incident investigation, it is confirmed the RCS control parameter on Feb. 28th was not appropriate. This indicates the possibility of insufficient verification of the process from creating the parameter to setting it on the satellite. This situation is under investigation.

- It is confirmed that the satellite had not been controlled by the thruster after resetting RCS control parameters on Feb. 28.

Figure 12: Presumed mechanism 3: From the attitude rotation to the rotation anomaly

• As the result of increase in rotation speed, parts which are vulnerable to the rotation, such as SAP, EOB and so on, might break up and separate off from the satellite main body.

• The main body of the satellite is spinning fast.

• Some parts which are vulnerable to the rotation, such as SAP, EOB and so on, might break up and separate off from the main body.

• The communication with the satellite cannot be established (since March 28). -The radio waves were received three times from ASTRO-H during 3/26 to 3/28, but the telemetry data were not obtained. JAXA continues the investigation to estimate the satellite status and recover its operation. Through the investigation, JAXA has recognized the following situation.

- Carrier frequency has been shifted to about 200 kHz from normal.

- Frequency spectral is different from the result of ground test.

• Decreasing the amount of Helium inSXS ( Soft X-ray Imager) refrigerator. (JAXA estimates that it is not depleted for now (as of April 15th).

• On April 1, 2016, JAXA held a press briefing to explain the status of the X-ray Astronomy Satellite "Hitomi"(ASTRO-H) and the JAXA team's activities to re-establish communications with "Hitomi." 26)

- The health check for all the scientific instruments (SXS, SXI, HXI, SGD) has been completed on March 26, 2016. The project was scheduled to proceed with the calibration phase in the middle of April. Observations for performance verification were conducted on March 25 & 26, 2016 as preparation for the calibration phase.

- The plan was to practice Command/Telemetry Operation in USC (Uchinoura Station Center) of JAXA and Ranging operation for trajectory determination in MSP (Maspalomas, Spain) or MGN (Mingenew, Australia). All the telemetry data including non-visible time have been obtained till USC operation, which ends at 03:02 on March 26. Only HK data during visible time have been obtained by MSP/MGN operation.

Figure 14: Hitomi Sequence of Events. This shows the observation plan and satellite tracking condition based on events, including the JSpOC information (image credit: JAXA)

• Receiving the telemetry data is our top priority to understand the current status of Hitomi

• After communication anomaly, JAXA ground tracking stations are assigned. In the effort of obtaining the telemetry data from Hitomi, JAXA continues to send the command to the estimated trajectory.

• JAXA has received radio signals 4 times as follows, but not received telemetry data yet, therefore the health status has not been confirmed.

- 3/26 23:49~23:52 (for 3 min) @USC,23:48~23:51(for 3 min) @ KTU

- 3/27 01:23~01:27 (for 4 min) @USC, 01:21~01:27 (for 6 min) @ KTU

- 3/28 22:06 (for about 10 sec) @ USC

- 3/29 00:33 (for about 6 sec) @ SNT

• As far as JAXA understands, no other satellite was present when JAXA received these 4 radio signals. JAXA believes it is likely that these signals came from Hitomi.

• JSpOC (Joint Space Operations Center) of the USAF announced five pieces were separated from HITOMI, but JAXA confirmed the trajectory of two pieces.

• By backtracking the trajectory of the two objects, it is confirmed that they were on almost the same trajectory as Hitomi at 10:37 on 26 March. That shows that the two objects are from the Hitomi satellite.

• The estimated break-out time by JAXA is consistent with the time (10:42±11M), JSpOC reported.

Table 8: Observation results by radar or telescope

• The first priority is to re-establish communications with Hitomi. JAXA is making the best effort for establishing the communication with Hitomi by sending the radio signals with commands by using the most possible chances of JAXA ground stations in Japan as well as in foreign countries.

• And also the detailed analysis and recovery schemes on the following events in investigating by analyzing the obtained telemetry data, estimating the state of Hitomi, conducting fault tree analysis, etc.

• March 27, 2016: JAXA found that communication with the X-ray Astronomy Satellite "Hitomi" (ASTRO-H), launched on February 17, 2016 (JST), failed from the start of its operation originally scheduled at 16:40, Saturday March 26 (JST). Up to now, JAXA has not been able to figure out the state of health of the satellite. While the cause of communication failure is under investigation, JAXA received short signal from the satellite, and is working for recovery. 27)

- Under theses circumstances, JAXA set up emergency headquarters, headed by the President, for recovery and investigation. The headquarters held its first meeting today, and has been working for recovery and the investigation of the cause.

• February 29, 2016: JAXA confirmed the completion of a sequence of important operations of the X-ray Astronomy Satellite "Hitomi" (ASTRO-H), including turning the cooling system on, test operation of the SXS (Soft X-ray Spectrometer), and extending the EOB (Extensible Optical Bench). With this confirmation, the critical operation phase of Hitomi is completed. 28)

- The operations team turned on the SXS cooling system for test operations after the launch on February 17. The team then confirmed that it reached the absolute temperature of 50 millidegrees (0.050 K, or -273.1ºC) on February 22.

- JAXA will perform the initial functional verification of the onboard instruments for about one and half months, then the team will conduct calibration observations for another 6 weeks.

• Feb. 17, 2016: JAXA confirmed that ASTRO-H satellite has deployed its SAPs (Solar Array Paddles) nominally through data transmitted from the satellite and received at the Uchinoura Ground Station at 7:40 PM JST (10:40 GMT) on February 17, 2016. The satellite is currently in good health. 29)

Sensor complement: (SXS, SXI, HXT, HXI, SGD, CAMS)

The ASTRO-H mission carries a set of four different instruments: two hard X-ray images (HXI) with dedicated hard X-ray telescopes, one micro-calorimeter (SXS) and one CCD camera (SXI), each behind dedicated soft X-ray telescopes. A non-focusing detector will also cover the soft gamma-ray range up to 600 keV (SGD). All instruments are co-aligned and will operate simultaneously. The ASTRO-H Quick Reference document is also available in PDF format for your convenience. 30)31)

ASTRO-H instruments include a high-resolution, high-throughput spectrometer sensitive over 0.3-12 keV with high spectral resolution of ΔE ≤ 7 eV, enabled by a micro-calorimeter array located in the focal plane of thin-foil X-ray optics; hard X-ray imaging spectrometers covering 5-80 keV, located in the focal plane of multilayer-coated, focusing hard X-ray mirrors; a wide-field imaging spectrometer sensitive over 0.4-12 keV, with an X-ray CCD camera in the focal plane of a soft X-ray telescope; and a non-focusing Compton-camera type soft gamma-ray detector, sensitive in the 40-600 keV band. The FOVs and effective areas of these instruments are shown in Figures 16 and 17. The simultaneous broad bandpass, coupled with high spectral resolution, will enable the pursuit of a wide variety of important science themes.

Figure 16: FOVs of instruments SXS, SXI and HXI as well as FWHM FOV of a SGD fine collimator (image credit: JAXA/ISAS)

Figure 17: Effective area for the different instruments onboard ASTRO-H (image credit: JAXA/ISAS)

SXSs (Soft X-ray Spectrometers)

The flight SXTs (Figure 18) were fabricated at NASA/GSFC and have been delivered to JAXA. According to calibration at GSFC and ISAS, the angular resolution (Half Power Diameter : HPD) is 1.3 arcmin and 1.2 arcmin for SXT-1 and SXT-2, respectively. The result obtained with SXT-2 exceeds the desired goal. Effective areas are measured to be ~590 cm2 at 1 keV and ~430 cm2 at 6 keV. In order to collect more photons, SXT-2, which has a slightly better angular resolution, will be used for the SXS, because it has a smaller detector area. Ground calibration for SXT-1 and SXT-2 has been completed. All the basic performance characteristics, effective area (on/off-axis), PSF (on/off-axis), and stray light have been measured during the calibration.

The SXS instrument is a combination of a SXT (Soft X-ray Telescope), the XCS (X-ray Calorimeter Spectrometer) and the cooling system.

ASTRO-H has two identical SXTs (Soft X-ray Telescopes). One is for SXS (SXT-S) and the other for SXI (SXT-I). SXT is similar in concept to the X-Ray Telescope (XRT) of Suzaku (ASTRO-E2). It is composed of 203 thin reflector shells tightly nested confocally and coaxially. A conical approximation of Wolter-I type optics is used.

Instrument

No of telescopes

Focal length

Diameter

No of nested shells

HPD (Angular resolution)

Aeff (on-axis effective area)

SXT

2

5.6 m

45 cm

203

1.3 arcmin

560 x 425 cm2

XRT-I

4

4.75 m

40 cm

175

2.0 arcmin

470 x 320 cm2

Table 11: Comparison of ASTRO-H/SXT and Suzaku/XRT

Three different thicknesses of Al substrates (152, 229, and 305 µm) are used, in which the outer shells use the thicker substrates. This is intended for achieving a large collecting area with a better imaging quality than Suzak/XRT. The considerably better HPD than XRT is realized by fixing the reflectors to support bars with adhesive. Stray light, or contaminating X-rays from outside of the field of view, is reduced by a stray-light baffle, which consists of coaxially-nested cylindrical aluminum blades placed above each reflector. A thermal shield is attached in front of the pre-collimator to stabilize the thermal environment of the SXT. The shield is made of Al-coated polyimide film to ensure a large effective area in the soft energy band.

XCS (X-ray Calorimeter Spectrometer): The XCS is a 32 channel system with an energy resolution of 7 eV between 0.3-12 keV. The XCS is placed at the focal length of 5.6 m. The effective area is optimized at high energies, with 210 cm2 at 6 keV, 50 cm2 at 0.5 keV, and 160 cm2 at 1 keV. The energy range will span from 0.3 to 12 keV with a superb energy resolution of 7 eV (specifications) at 6 keV.

The goal is to reach 4 eV spectral resolution, which would allow a spectral power of R=1600 at 6 keV (current best estimate is 5 eV). In contrast to gratings, the spectral power increases as a function of energy. The microcalorimeter will be an array of 6 x 6 pixels of pixel size 814 µm x 814 µm, operating at 50 mK, with a FOV (Field of View) of 3.05 arcmin x 3.05 arcmin (i.e., a pixel scale of 30 arcsec/pixel) and angular resolution of 1.7 arcmin half-power diameter (goal of 1.3 arcmin; the current best estimate already reaches this value).

With microcalorimetry, X-ray photons strike the detector's absorbers and their energy is converted to heat, which a thermometer then measures. The heat is directly proportional to the X-ray's energy, which can reveal much about the physical properties of the object emitting the radiation. To measure as many X-ray photons as possible, Richard Kelly and hid GSFC team placed an array of microcalorimeter detectors at the focus of a large X-ray telescope. The detector package and cooling system then were placed inside a dewar. 33)

The XCS will also carry a suite of filters: X-ray filters to attenuate the X-ray signal for bright X-ray sources (one Be-type to block the low-energy photons and one neutral density for very bright sources and background calibration), and one polyimide filter to avoid contamination in early phases of the mission. Two open positions are also available. In addition, one filter wheel position with a 55Fe source can be inserted for backup calibration purposes. Novel Modulated X-ray Sources are also mounted below the filter wheel for calibration as well. The filters are necessary not only to avoid saturation, but also because the energy resolution depends on the X-ray count rate. Energy event grades are defined, with the best calorimeter resolution obtained only for high-resolution (HR) and medium-resolution (MR) events. Finally, the required time assignment resolution of the SXS is 80 ms, whereas the maximum counting rate should reach 150 count/s/pixel.

Cooling system: SXS is a cryogenic spectrometer array with ≤7 eV energy resolution in 0.3-12 keV (Figure 20). The cooling system is designed to cool the array to 50 mK and regulate the temperature to within 2 µK rms. From the detector stage to room temperature, the cooling chain comprises a 3-stage Adiabatic Demagnetization Refrigerator, superfluid liquid 4He, a 4He Joule-Thomson cryocooler and 2-stage Stirling (2ST) cryocoolers. 34)

SXI is equipped with X-ray CCD devices placed at the focal plane of the Soft X-ray Telescope (SXT-I). The SXI has an imaging-spectroscopic capability of a wide field (38 arcmin) and a medium energy resolution (E/ΔE~40 @ 6 keV) at the soft X-ray band (0.4-12 keV). The SXI is a successor of the XIS onboard the Suzaku observatory.

Instrument

No of CCD

Layout

Pixel scale

Format
(1 CCD)

FOV
(arcmin)

Aeff (cm2)

Grasp (cm2 arcdeg2)

Frame time (s)

Illumination

Type

Depl. layer
(mm)

SXI

4

2 x 2 array

1.74"

640 x 640

38 x 38

214 x 360

40

4

BI x 4

p-channel

200

XIS

4

4 coaligned

1.04"

1024 x 1024

18 x 18

240 x 950

35

8

FIx3, BIx1

n-channel

80 (FI), 45 (BI)

Table 12: Comparison of ASTRO-H/SXI and Suzaku/XIS

Mechanical coolers will be used to keep the device temperature at -120º C. The p-channel CCD will have a thick depletion layer to extend the hard-band coverage. The thick depletion layer also leads to a significant background reduction above ~7 keV compared to the same BI CCD in Suzaku/XIS. The BI devices is more resistant than the FI devices to micro-meteorite hits, which made a part of the XIS dysfunctional. The design is underway to suppress contamination accumulation on the CCD chips. Onboard calibration source is under discussion.

The 2x2 CCD array covers a very large FOV. The nominal position is placed at the center of SXS FOV, which is 4.3 arcmin offset from the SXI center. A gap of ~20 arcsec is between the chips. Similarly to XIS, the background level of SXI is expected to be low and stable, benefiting from the low-earth orbit of the satellite.

The HXT is designed based on the InFOCµS and SUMIT balloon-borne experiments (Nagoya University, Osaka University and JAXA) under the constraint of the space within the nose fairing of the H-IIA rocket. Two identical HXTs are mounted on the top plate of the FOB (Fixed Optical Bench), while the HXIs (Hard X-ray Imagers), which are the focal plane detectors of the HXTs, are placed on the HXI plate of the EOB (Extensible Optical Bench). The focal length of 12 m will be realized by extending the EOB. Figure 22 shows the configuration of XRTs (X-ray Telescopes) on the FOB top plate, including the soft X-ray telescopes (SXT-I and SXT-S). Each XRT has a sunshade to block direct X-rays from the Sun. The FOB top plate is covered with MLI (Multilayer Insulation) except for the aperture area of the XRTs. 35)

Figure 22: Configuration of the X-ray telescopes on the FOB top plate. The FOB top plate is covered with MLI (shown transparent in this figure), image credit: ASTRO-H consortium

Figure 23 shows a schematic view of the current design of the HXT. The HXT has three mount tabs to be mounted on the FOB top plate. The HXT consists of four parts: two thermal shields, the pre-collimator, and the reflectors. The thermal shields are made of a PET (Polyethylene Teleftalate) film as thin as 5 µm, coated with an aluminum layer with a thickness of 30 nm, and is set on top of the HXT in order to thermally isolate the HXT mirror from space. A thermal analysis of the HXT predicts that the temperature of the secondary reflectors will be lower than the operating temperature of the HXT in orbit due to a radiation coupling between the reflectors and the inside of the satellite. Hence, a thermal shield on bottom of the HXT will be installed. The pre-collimator (or stray-light baffle) consists of thin cylindrical shells, which are called blades.

The Suzaku pre-collimators were mechanically independent from the mirror housing, but in the ASTRO-H HXT, the pre-collimator is integrated into the mirror housing in order to reduce stray light efficiently without any loss of the on-axis effective area. The HXT mirror employs conically-approximated thin-foil Wolter-I optics. The approximated parabolic and hyperbolic foils are called primary and secondary reflectors in Figure 23, respectively. These reflectors are held at their desired location and confocally aligned by the grooves of alignment bars on each top and bottom edge of the reflectors. The diameters of the innermost and the outermost reflectors are 120 mm and 450 mm, respectively. The incident grazing angles of the reflectors range from 0.07º to 0.27º. Although the incident grazing angle at the outermost radius is 0.27º, which corresponds to the critical grazing angle of platinum for total reflection of the 18 keV photon, the reflectivity in hard X-rays with E > 20 keV is enhanced due to Bragg reflection (Figure 24).

To achieve high aperture efficiency despite the small incident angle, the thickness of the reflector substrate should be reduced. Thus, a 0.2 mm aluminum substrate is used. The reflector shells are confocally nested with maximal tightness. In the Suzaku XRT, thin foil substrates with a slant length of 100 mm were used. Instead, thin substrates with 200 mm slant length are selected for the HXT to reduce the total number of nestings and to obtain high aperture efficiency. As the result, the total number of the nesting shells is determined to be 213.

Figure 23: Schematic view of the current design of HXT. The right part displays a cross-section of the HXT (image credit: ASTRO-H consortium)

Reflectors: The reflectors are fabricated by the epoxy-replication method in which a thin depth-graded Pt/C multilayer is sputtered onto the smooth surface of a glass tube and transferred to a conically shaped aluminum substrate with epoxy glue. The basic technology for fabricating the ASTRO-H/HXT has been established through the balloon-borne experiments, " InFOCµS" and " SUMIT".

The aluminum foils are cut into a fan shape and stacked onto a shaping mandrel. The foils are pressed onto the mandrel with air pressure and formed into a precise conical shape at 200ºC for 12 hours in an oven. The shaping mandrel has a small figure error which is less than 6 µm peak-to-bottom, and the shaped substrate has the same error. Cone angles of the substrates range from 0.07º - 0.27º for primary reflectors and from 0.21º - 0.81º for the secondary ones.

For Bragg reflection, reflectivity is enhanced when the Bragg condition is satisfied; n λ = 2d sin(θ), where n is the order of reflection, λ the wavelength of the incident X-ray, d the periodic length of the multilayers, and θ the grazing angle of incidence. A simple multilayer (with constant d) shows a narrow energy/angular response. For astronomical applications, the narrow response is broadened by stacking multi-layers with different sets of periodic length and number of layer pairs in the depth direction, which produces the "supermirror". A supermirror is designed so that the periodic length decreases from the top surface to the base substrate. X-rays with higher energy, which have a longer penetration depth, are reflected by a deeper layer. Because the reflectivity response strongly depends on the incident angle, the multilayer design has been optimized for each reflector group, which is defined by the range of grazing incident angles.

Table 14 describes the grouping definition. The range of a group is determined with roughly 10% of an incident angle. Figure 24 shows a reflectivity versus X-ray energy. Groups 1 through 7 are designed to show a flat curve up to 78 keV which corresponds to the Pt-K edge energy. Reflectivity of Group 8 and higher falls at an energy lower than 70 keV because the multilayer parameters are designed for these groups with the limit of minimum d-spacing of 24 Å, which is the manufactural limit with low roughness. Figure 25 shows a total effective area and contribution of each group estimated from the geometrical reflector parameters and the reflectivity of the supermirror with an assumption of interfacial roughness of 3 Å (Debye-Waller factor).

Figure 24: Reflectivity curve of each group; definition of the group is described in Table 14 (image credit: ASTRO-H consortium)

Figure 25: Total effective area (black) and contributions of each group (color), image credit: ASTRO-H consortium

Pre-collimator: Because of the grazing incident optics used in the HXT, some off-axis X-rays are also reflected on the supermirror surface and then reach the focal plane without the normal double reflection. These X-rays create a ghost image in the detector FOV (Field of View) , called stray light. The origin of stray light may be a bright point source located outside of the telescope FOV or a diffuse source extended over the telescope FOV, such as cluster of galaxies or the Cosmic X-ray Background. The ghost image by stray light acts as a diffuse background so that the stray light causes degradation of the detection limit. In order to reduce the stray light, some X-ray telescope systems are equipped with a baffle structure (pre-collimator) in front of the mirrors. The pre-collimator design was first introduced to the Suzaku XRT.

The reduction rate of the stray light depends on the height, thickness and material of the blades. Since the pre-collimator mount makes the telescope vignetting narrower, there is a trade-off between stray-light reduction and the telescope FOV. Thus, the blade properties need to be optimized. The current design parameters of the HXT pre-collimator are: The blade thickness and height (measured from the top edge of the primary foils) are 150 µm and 50 mm, respectively. This blade height is adequate to eliminate the secondary reflection from > 20 arcsec off-axis from the HXI FOV (32 mm x 32 mm). The blade material selected is aluminum.

TS (Thermal Shield): In the HXT, a TS is used to cover the entrance side of the telescope housing as in Suzaku. The main purpose of the TS together with the heater attached to the HXT housing is to keep the HXT mirror temperature within a specified range. The HXT-TS also works to block optical light from the sky and from the surface of Earth illuminated by the Sun. The performance requirements for the HXT-TS are summarized as follows:

10) HXT-TS should have a thermal control function, which meets the temperature condition for HXT mirror performance.

11) HXT-TS does not significantly reduce the low energy detection efficiency of the HXI.

In order to give adequate thermal control function to the TS, an aluminum-coated plastic film is used, which has low solar absorptance and a low infrared emissivity. Since the energy band of the HXI covers 5 keV at the lower end, a 5 µm PET equivalent thin film with high mechanical strength as HXT-TS. The X-ray transmission of 5 µm PET is 98 %. In order to give enough mechanical strength to survive in various environments, a stainless steel mesh with a wire pitch, width, and thickness of 3 mm, 0.1 mm and 0.25 mm, respectively, is used to support the thin film. The transmission of soft X-rays down to 8 keV is 94%, which maintains the effective area at 8 keV.

In addition to the improvement of sensitivity brought by hard X-ray optics, the HXI provides a true imaging capability which enable us to study spatial distributions of hard X-ray emission.

The HXI detectors use the technology of Si strip detectors (four layers) to absorb the soft X-rays below 30 keV, and include one layer of CdTe strip to absorb the hard X-rays (20-80 keV). The HXI will provide images and spectra in the 5-80 keV range with moderate energy resolution (0.9 keV at 14 keV in the double-sided Si layers, 1.5-1.7 keV in the double-sided CdTe layer). The effective area is about 300 cm2 at 30 keV. The angular resolution in the HXI is 1.7 arcmin for a field of view of 9 arcmin x 9 arcmin. A BGO (Bismuth Germanate,Bi4 Ge3 O12) shield will work as veto counter and suppress a significant amount of background. The fast timing response of the HXI detectors will allow bright sources to be observed without pile-up. The HXI will operate in one single mode, but users will be able to select events in all HXI layers or in the top Si strip layer for faint sources (Ref. 30).

Legend to Figure 27: Each DSSD has a size of 3.2 x 3.2 cm2 and a thickness of 0.5 mm, resulting in 2 mm in total thickness, the same as that of the PIN detector of the HXD onboard Suzaku. A CdTe strip detector has a size of 3.2 x 3.2 cm2 and a thickness of 0.75 mm.

The sensor part of the HXI consists of four layers of 0.5 mm thick DSSD (Double-sided Silicon Strip Detectors) and one layer of 0.75 mm thick CdTe imaging detector (Figure 27). In this configuration, soft X-ray photons below ~ 20 keV are absorbed in the Si part (DSSD), while hard X-ray photons above ~20 keV go through the Si part and are detected by a newly developed CdTe double-sided cross-strip detector. The E< 20 keV spectrum, obtained with the DSSD Si detector, has a much lower background due to the absence of activation in heavy material, such as Cd and Te.

The DSSDs cover the energy below 30 keV while the CdTe strip detector covers the 20{80 keV band. In addition to the increase in efficiency, the stack configuration and individual readouts provide information on the interaction depth. This depth information is very useful to reduce the background in space applications, because we can expect that low energy X-rays interact in the upper layers and, therefore, it is possible to reject the low energy events detected in lower layers. Moreover, since the background rate scales with the detector volume, low energy events collected from the first few layers in the stacked detector have a high signal to background ratio, in comparison with events obtained from a monolithic detector with a thickness equal to the sum of all layers.

In the energy band above 10 keV, the number of photons from the source decreases and the detector background becomes the major limitation to its sensitivity. Since a significant fraction of the background events originate from interactions of the cosmic-ray with the detector structure, a tight active shield to reject cosmic-ray induced events is critical. Fast timing response of the silicon strip detector and CdTe strip detector allows us to place the entire detector inside a very deep well of an active shield made of BGO (Bi4Ge3O12) scintillators. The signal from the BGO shield is used to reject background events.

SGD (Soft Gamma-ray Detector)

The SGD instrument measures soft γ-rays via reconstruction of Compton scattering in the Compton camera, covering an energy range of 40 - 600 keV with a sensitivity at 300 keV: 10 times better than that of the Suzaku Hard X-ray Detector. It outperforms previous soft γ-ray instruments in background rejection capability by adopting a new concept of a narrow-FOV Compton telescope, combining the Compton cameras and active well-type shields.

Figure 28 shows the conceptual design of the Si/CdTe semiconductor Compton camera, together with two types of shields; one is a BGO active shield and the other is a fine collimator made of PCuSn.

In the Si/CdTe Compton camera, events involving the incident gamma-ray being scattered in the Si detector and fully absorbed in the CdTe detectors are used for Compton imaging. The direction of the gamma-ray is calculated by solving the Compton kinematics with information concerning deposit energies and interaction positions recorded in the detectors. In principle, each layer could act not only as a scattering part but also as an absorber part. A very compact, high-angular resolution (fineness of image) camera is realized by fabricating semiconductor imaging elements made of Si and CdTe, which have excellent performance in position resolution, high-energy resolution, and high-temporal resolution. As shown schematically in Figure 28, the detector consists of 32 layers of 0.6 mm thick Si pad detectors and eight layers of CdTe pixellated detectors with a thickness of 0.75 mm. The sides are also surrounded by two layers of CdTe pixel detectors.

Figure 28: Conceptual drawing of an SGD Compton camera unit (green sections are the BGO anti-coincidence shields, red planes are the Si strip detectors in which the Compton scattering, occurs and the blue parts are the CdTe section in which the photons are absorbed). In order to further restrict the FOV and to reduce contamination from the CXB (Cosmic X-ray Background) for photons below 100 keV, a fine collimator is installed (image credit: ASTRO-H consortium)

The camera is mounted inside the bottom of a well-type active shield (Figure 29). The major advantage of employing a narrow FOV is that the direction of incident γ-rays is constrained to be inside the FOV. If the Compton cone, which corresponds to the direction of incident gamma-rays, does not intercept the FOV, we can reject the event as background. Most of the background can be rejected by requiring this condition. The opening angle provided by the BGO shield is ~10º at 500 keV. An additional PCuSn collimator restricts the FOV of the telescope to 300 for photons below 100 keV to minimize the flux due to the cosmic X-ray background in the FOV. Since the scattering angle of gamma-rays can be measured via reconstruction of the Compton scattering in the Compton camera, the SGD is capable of measuring polarization of celestial sources brighter than a few x 1/100 of the Crab Nebula, if they are polarized by more than 10%. This capability is expected to yield polarization measurements in several celestial objects, providing new insights into properties of soft gamma-ray emission processes.

In 2011, the technology behind the SGD was demonstrated in measurements of the distribution of Cs-137 in the environment of Fukushima. In addition to showing that the SGD technology works as designed "in the field" the use of the Si/CdTe Compton Camera provided crucial information in understanding how the fallout from the 2011 nuclear accident was distributed.

Legend to Figure 29: The SGD consists of two identical sets of an SGD-S (Sensor) which are called SGD-1 and SGD-2. The SGD-S is a detector body that includes a 3 x 1 array of identical Compton Camera Modules surrounded by BGO shield units and fine passive collimators. The two SGD-S are mounted on opposite sides of the spacecraft side panels to balance the load, since each has a large mass of 150 kg.

CAMS (Canadian Astro-H Metrology System)

CAMS is provided by CSA (Canadian Space Agency) and developed by the Neptec Design Group Ltd, Kanata, Ontario. In return for CAMS, three Canadians will be part of the mission's science team and have privileged access to the space observatory to study black holes, supernovas, and galaxy clusters. They will also investigate how galaxies like our own Milky Way were formed and how matter behaves under extreme conditions. Luigi Gallo of Saint Mary's University leads Canada's science team, which includes Brian McNamara of the University of Waterloo and Samar Safi-Harb of the University of Manitoba, as well as their respective teams of researchers and students. The Canadian science team will have access to the proprietary data during the first year of the observatory's operations, after which the entire Canadian astronomy community will have opportunities to propose ideas for further space-based investigations. 36)

CAMS is a laser alignment system that is designed to measure the displacement of the HXI (Hard X-ray Imager) relative to the rest of the satellite. This is to account for any distortions in the EOB (Extensible Optical Bench) to which the HXI is attached, which will provide its focal length. CAMS consists of a laser and detector module (CAMS-LD) located on the top plate of the FOB, and a corner cube mirror (a passive target module CAMS-T), located at the opposite end on an extendable mast, 12 m away (Figure 31). The mast tends to bend and twist due to the extreme temperature variations in space. CAMS will precisely measure the mast's distortions to a level of accuracy equivalent to the width of two human hairs. 37)

Two metrology systems will be installed inside the spacecraft's main body. Each metrology system is capable of measuring absolute lateral displacements of the optical bench to an accuracy of at least 60 µm. By comparing the measured lateral displacement on each detector, the rotation about the telescope's main axis can be determined and image correction performed. 38)

Figure 31: Illustration of the CAMS onboard ASTRO-H. A corner cube optical reflector installed at the end of the space observatory's 6 m mast where the hard X-ray telescope sensors are located (image credit: CSA)

Legend: The CAMS sensor units fire laser beams onto the corner cube, which reflect the beam back into the unit where lateral displacements are measured down to a level of accuracy equivalent to the width of two human hairs. By comparing the measured lateral displacement on each sensor, the rotation about the telescope's main axis can be determined and the data acquired at the time of observation can be calibrated to a much higher level of accuracy than in the absence of the Canadian measurement system.

Expected scientific performance

ASTRO-H mission is expected to revolutionize high energy science on all astronomical scales. These include: 1) the very compact scales around black holes, 2) high-temperature plasmas around stars at all stages of their lives, 3) the diffuse hot media supernova remnants, 4) the interstellar media of galaxies, 5) clusters of galaxies and 6) the large scale diffuse inter galactic medium. Opening up new parameter space in i) energy range, ii) sensitivity, iii) spectral resolution and iv) polarimetry, ASTRO-H will allow detailed study of the dynamics, composition, morphology and evolution of matter across these cosmic scales.

In order to demonstrate the new science accessible with ASTRO-H, the project is preparing a series of white papers by dedicated task forces. The categories of the task forces are listed in Table 16.

The high energy resolution of the non-dispersive SXS (Soft X-ray Spectrometer) is unique in X-ray astronomy, since no other previously or currently operating spectrometers could achieve a comparable high energy resolution, high quantum effciency, and spectroscopy for spatially extended sources at the same time. Imaging spectroscopy of extended sources can reveal line broadening and Doppler shifts due to turbulent or bulk velocities. This capability enables the determination of the level of turbulent pressure support in clusters, SNR ejecta dispersal patterns, the structure of AGN (Active Galactic Nuclei) and starburst winds, and the spatially dependent abundance pattern in clusters and elliptical galaxies. The SXS can also measure the optical depths of resonance absorption lines, from which the degree and spatial extent of turbulence can be inferred. Additionally, the SXS can reveal the presence of relatively rare elements in SNRs and other sources through its high sensitivity to low equivalent width emission lines. The low SXS background ensures that the observations of almost all line-rich objects will be photon limited rather than background limited.

All studies of the total energy content of cosmic plasma (including that of non-thermal particles), aimed to draw a more complete picture of the high energy universe, require observations by both a spectrometer capable of measuring the bulk plasma velocities and/or turbulence with the resolution corresponding to the speed of a few x 100 km/s and an arcmin imaging system in the hard X-ray band, with sensitivity two orders of magnitude better than previous missions (Figure 34). The high energy resolution provided by SXS, will make it possible to detect dozens of emission lines from highly ionized ions and measure, for the first time, their line profiles with sufficient accuracy to study gas motions. The HXI (Hard X-ray Imager) will extend the simultaneous spectral coverage to energies well above 10 keV, which is critical for studying both thermal and non-thermal gas in clusters. In bright, nearby galaxy clusters, such as the Perseus Cluster, ASTRO-H will determine the projected velocity vbulk (line centroid) and the line-of-sight velocity dispersion σv (line width) as a function of position, providing a measure of the bulk and small scale velocities of the plasma.

XMM-Newton and Suzaku spectra of AGN frequently show time-variable absorption and emission features in the 5-10 keV band. If these features are due to Fe, they represent gas moving at very high velocities with both red- and blue-shifted components from material presumably near the event horizon. The sensitivity of the SXS in the Fe-K region extends the observed absorption measure distribution of the outflow up to the highest ionization states accessible. Due to the high-resolution and sensitivity, it will also be able to provide the definitive proof for the existence of ultra-fast outflows, and if so, characterize their physical properties in great detail. These ultra-fast outflows carry very large amounts of energy and momentum, and are of fundamental importance for feedback studies. The ASTRO-H SXS observations of highly ionized outflows will measure the velocity, density and dynamics of the material, revolutionizing our understanding by determining the launch radius of these winds. A simulated spectrum for NGC 4151 is shown in Figure 33.

The imaging capabilities at high X-ray energies will open a new era in high spatial resolution studies of astrophysical sources of non-thermal emission above 10 keV, probed simultaneously with lower energy imaging spectroscopy. This will enable us to track the evolution of active galaxies with accretion flows which are heavily obscured, in order to accurately assess their contribution to black hole growth over cosmological time. It will also uniquely allow mapping of the spatial extent of the hard X-ray emission in diffuse sources, thus tracing the sites of cosmic ray acceleration in structures ranging in size from mega parsecs, such as clusters of galaxies, down to parsecs, such as young supernova remnants. Those studies will be complementary to the SXS measurements: observing the hard X-ray synchrotron emission will allow a study of the most energetic particles, thus revealing the details of particle acceleration mechanisms in supernova remnants, while the high resolution SXS data on the gas kinematics of the remnant will constrain the energy input into the accelerators.

Figure 33: Simulated spectrum for NGC 4151 for 100 ks exposure time. The same flux condition of the 2002 HETGS spectrum60 is used for simulation (image credit: ASTRO-H consortium)

Figure 34: Left: The 3σ sensitivity curves for the HXI and SGD onboard ASTRO-H for an isolated point source. (100 ks exposures and ΔE=E = 0:5); Right: Differential sensitivities of different X-ray and γ-ray instruments for an isolated point source. Lines for the Chandra/ACIS-S, the Suzaku/HXD (PIN and GSO), the INTEGRAL/IBIS (from the 2009 IBIS Observer's Manual), and the ASTRO-H/HXI,SGD are the 3σ sensitivity curves for 100 ks exposures. A spectral bin with ΔE=E = 1 is assumed for Chandra and ΔE=E = 0:5 for the other instruments (image credit: ASTRO-H consortium)

As shown in Figure 34, the sensitivity to be achieved by ASTRO-H (and similarly NuSTAR) is about 2 orders of magnitude better than previous collimated or coded mask instruments that have operated in this energy band. This will bring a breakthrough in our understanding of hard X-ray spectra of celestial sources in general. With this sensitivity, 30-50% of the hard X-ray Cosmic Background will be resolved. This will enable us to track the evolution of active galaxies with accretion flows that are heavily obscured, in order to accurately assess their contribution to the Cosmic X-ray Background (i.e., black hole growth) over cosmic time.

There is a strong synergy between the hard X-ray imaging data and the high resolution (ΔE ≤7 eV) soft X-ray spectrometer: the kinematics of the gas, probed by the width and energy of the emission lines, constrains the energetics of the system. The kinematics of the gas provides information about the bulk motion; the energy of this motion is in turn responsible for acceleration of particles to very high energies at shocks, which is in turn manifested via non-thermal emission processes, best studied via sensitive hard X-ray measurements. All studies of the total energy content (including that of non-thermal particles), aimed to draw a more complete picture of the high energy universe, require observations by both a spectrometer capable of measuring the bulk plasma velocities and/or turbulence with the resolution corresponding to the speed of a few x 100 km/s and an arcmin imaging system in the hard X-ray band, with sensitivity two orders of magnitude better than non-imaging missions.

The power of ASTRO-H is that those gas dynamics can be probed both with micro calorimeters and the hard X-ray imaging instruments at the same time. Regarding the process of particle acceleration, the velocity field probed by the SXS data will tell us the conditions of the environment in which acceleration occurs, and the hard X-ray data will reveal how much acceleration is really taking place. Furthermore, the SGD data will tell us the maximum energy of the accelerated particles. In this way, ASTRO-H will give us a new view of the non-thermal processes taking place in the universe.

The information compiled and edited in this article was provided byHerbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (herb.kramer@gmx.net).